In the terminal, parse errors do not have a backtrace. But we still need
to initialize the result.bt field.

The heuristic in error_to_string which removes the backtrace below the
highest Core.eval call works in most cases, but will fail if
makedocs/doctest gets called in a more complicated context, as there may
sometimes be eval calls higher up in the stacktrace.

On the nightly, syntax errors now get stacktraces, which means that
their output is no longer deterministic. Hence we can't really compare
against the fixed output.